Loop equipped radio receiver



Sept. 30, 1941. cf TRAVI 2,257,250

LOOP EQUIPPED RADIO RECEIVER Filed May 16, 1940 Mp M Patented Sept. 30, 1941 LOOP EQUIPPED RADIO RECEIVER Charles Travis, Wyndmoor, Pa., assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application May 16, 1940, Serial No. 335,581

In Great Britain May 30, 1939 8 Claims.

This invention relates to radio receivers adapted for operation with loop antennas, and more particularly to means for substantially improving the sensitivity of radio receivers and the apparent Q, or ratio of reactance to resistance, of loop antennas.

In order to improve the portability of radio receivers and to simplify their installation, it has been found desirable to employ compact resonant loop antennas capable of being disposed within the cabinets or housings containing the radio receiver chassis. Due to the small physical size of loops capable of being mounted within the smaller radio cabinets, and to the proximity of the loop to the various metal parts comprising the chassis and other receiver components, the Q of such self-contained loops is generally very low in practice. Thus where a Q of 100 or more might be desirable it has been found that in practice the Q of the loop antenna circuit, par-.

ticularly at the low frequency end of the tunable wave band, may fall as low as or less. The importance of securing a high effective or apparent Q will be apparent when it is considered that both the selectivity of the input stage and the overall .gain of a loop-equipped receiver are direct functions of the Q of the loop antenna employed.

It has been found that the effective or apparent Q of a loop may be conveniently improved in a radio receiver of the superheterodyne type by introducing regeneration at the carrier fre quency between the output of the frequency converter tubeand the loop.

According to this invention, there is provided a radio receiver of the superheterodyne type which includes in cascade a resonant loop antenna circuit, a frequency converter stage, an intermediate frequency amplifier and coupling network therefor, and means for coupling a portion of said loop antenna circuit to said intermediate frequency coupling network, whereby the apparent Q of said loop circuit is increased.

The invention may be clearly understood by reference to the accompanying drawing, the single figure of which is a diagrammatic illustration of a portion of a superheterodyne radio receiver embodying the present invention.

Referring to the drawing, there is shown a resonant loop circuit which includes the loop antenna l and the tuning condenser 2. The voltage across the condenser 2 may be impressed on the input grid of the frequency converter tube V1 as shown. The oscillator circuits associated with the first and secondgrids of tube V1 and tuned by the variable condenser 3 are well known in the art and require no description. The intermediate frequency output of the converter tube V1 may be applied to the input of the intermediate frequency amplifier'tube V2 by means of a coupling network which comprises a transformer T having primary and secondary windings 4 and 5 tuned to the intermediate frequency. The output of the amplifier V2 may be supplied to a suitable signal utilization means 6, such as a second detector and audio frequency amplifier, which may include an automatic volume or gain control source, the output of which may be supplied via the lead I, the resistor 8, and the secondary winding 5 to the gain control electrode of the amplifier V2. If desired, onlya fraction of the AVG voltage developed may be employed, for example by providing a voltage divider network 8-9 from the junction of which the gain control or AVC voltage may be derived.

Now although the coupling network 4-5 is tuned to the intermediate frequency, it will be found that an appreciable amount of radio frequency energy is associated with the said coupling network, the effect being most pronounced for radio frequencies most nearly of the order of the intermediate frequency. In accordance with this invention, coupling is provided between the said coupling network and the resonant loop circuit whereby an overall regenerative action is effected, thereby improving both the gain and selectivity of the receiver.

In the particular embodiment illustrated, the resonant loop circuit comprises, in series, and starting from the common ground connection, the tuning condenser 2, the loop antenna I, the feedback connection I2, the feedback coil l0, and the by-pass condenser ll to ground. The feedback coil [0 may be coupled either to the primary 4 or to the secondary 5 of the intermediate frequency transformer T, or to both windings if desired. In one experimental model which gave very satisfactory results, the tuning range of the loop circuit extended from 540 to 1550 kilocycles, while the intermediate frequency was 455 kilocycles. 2 turns coupled to both the primary and the secondary, but more closely to the latter.

The maximum regenerative feedback occurs at the low frequency end of the tuning range, and

falls off gradually at the high frequency end of The feedback coil l0 consisted of selectivity of the. circuits .ahead of; the converter only at the signal (R. F.) frequency, intermediate frequency feedback can readily be avoided by inserting anI. F. trap circuit in the feedback path 12. Similarly, if it be desired to provide feedback only at the intermediate frequency this can be accomplished by inserting in the path l2 a band pass filter designed to transmit only signals of the intermediate frequency.

By connecting the low potential end of the feedback coil 16 to the A. V. C. voltage source,

- as illustrated, the gain-controlling voltage may make the performance more uniform over the 7 tuning range of the receiver.

The manner in which the regeneration varies;

over the desired wave band 'rnay be controlled 1 in a number of ways. If the transformer T is double tuned and less than critically coupled its selectivity will be relatively high, and as a result the desired feedback will be appreciable only very near the 540 kilocycle end of the band. This narrowing of the band over which feedback is effective may be made even vmore pronounced by coupling the coil l0 substantially only to the sec ondary winding 5-. On the other hand. if the transformer T has a wide band-pass characteristic the decrease in regeneration with increasing signal frequency may be made to take place more gradually. V

In addition to the regenemtive feedback of radio frequency energy above referred to, it may U also be pointed out that an appreciable feedback of intermediate frequency energy also takes place, and this effect is also greatest at the low frequency endof thetuning range, for; at this end of the range the resonant frequency of the loop circuit will most closely approach the intermediate frequency. Only one feedback path l2 needbe employed for feeding back signals of both the incoming radio frequency and of the in-' termediate frequency, and the same coupling coil I0 is effective at both frequencies. The relative magnitudes of the R. F2. and I. F. feedback voltages will depend'upon :theoverall, design of the circuits and upon the gain of the tube V1. Thus although the invention is primarily directed to a means for improving the effective Q of theloop antenna circuits it is also contemplated, Where desired, to make use of I. F. feedback through the converter tube V; as explained above. I

In the particular circuit shown the radio frequency feedback is regenerative, while the intermediate frequency feedback is degenerative. If

both were regenerative the circuitmigh-t tend toward instability, particularly where the two feedback frequencies differed by only a relatively small amount, as.they do whenthelocp circuit is tuned to 540.kc., the intermediate frequency being constant at 455 kc. The effect of; I. F. degeneration is to reduce the apparent Q ofthe intermediate frequencytransfcrmer T, while the effect of R. F. regeneration is to. increase the apparent Q (decrease the effective losses) of the loop antenna l. Accordingly, in this particular embodiment the circuit employed 'tends' to increase'the selectivity and sensitivity of the loop circuit (and hence the gain of the antenna stage) .while' sl'ightly reducing :the gain andfselectivity of the I, F. amplifier stage] This as desirable condition since] it isIdesirable, to increase the tube il'l Q1de1 fto discriminate more; effectively against. image;- frequencies it being well known;

i s desired to provide feedback;

be applied to the input grid of the frequency converter tube V1 by Way of the connection 12 and the loop I. Specifically the connecting means between the A. V. C. voltage, source (which is included in the rectangle 6) and the input grid of the-converter V1 consists of a path which comprisesthe following elements and junction points: conductor 7, resistor 8, junctions l3, l4, and I5, coil l0; conductor I2, and the loop winding I. This connection eliminates the need for a separateA. V. C. connection to the gain controllingv element of the tube V1 and reduces the costof manufacture accordingly. It should be 'understood however that the invention is not restricted:

to the use of any particular gain control connections. f Y

Althoughthe invention may be applied to any employ small and relatively inefficient loop antennasv Moreover, although the invention has been described with particular referenceto the embodiment,v illustrated, it will be understood that the invention is capable of vario-us forms of physical expression, and is therefore, not to' be limited to the specific disclosure.

I claim: i

1. A radio receiver of the superheterodyne type comprising in cascade, a resonant loop antenna circuit, said circuit having atleast a' loop .antenna winding and a variable condenser for tuning said circuit overv apredetermined band of radio frequencies, a frequency, converter stage providing an intermediate frequency signalwhich lies outside of and below said band of frequencies, an intermediate frequency coupling network and an intermediate frequency amplifier, and a radio frequency feedback path connectedbetween the output circuit of said frequency converter stage and said loopantenna .circuitfor'producing retenna winding and a variable-condenser'for tuning saidcircuit over a predetermined band of radio frequencies, a frequency converter stage providingan intermediate'frequency signal which lies outside of and below said bandof frequencies, an intermediate frequency coupling network and an intermediate frequency amplifier, said intermediate frequency coupling network comprising at least: one winding tuned to theintermediatefrequency, and a radio frequency feedback path connected between the output circuit of said fre-'. quency converterstage and said loop antenna circuit for producing regeneration at said radio frequencies, saidnetwork forming anelement in said feedback path, said feedback path being constructed and arranged to provide maximum feedback at the low frequency end of the said band of frequencies, said feedback decreasing substantially as the high frequency end of the said band is approached.

3. A radio receiver of the superheterodyne type comprising in cascade, a resonant loop antenna circuit, said circuit having at least a loop antenna winding and a variable condenser for tuning said circuit over a predetermined band of radio frequencies, a frequency converter stage providing an intermediate frequency signal which lies outside of and below said band of frequencies, an intermediate frequency coupling network and an intermediate frequency amplifier, said intermediate frequency coupling network comprising a double tuned transformer, and a radio frequency feedback path connected between the output circult of said frequency converter stage and said loop antenna circuit for producing regeneration at said radio frequencies, said transformer functioning as an element in said feedback path, said feedback path bein constructed and arranged to provide maximum feedback at the low frequency end of the said band of frequencies, said feedback decreasing substantially as the high frequency end of the said band is approached.

4. A radio receiver of the superheterodyne type comprising in cascade, a resonant loop antenna circuit, said circuit having at least a loop antenna winding and a variable condenser for tuning said circuit over a predetermined band of radio frequencies, a frequency converter stage providing an intermediate frequency signal which lies outside of and below said band of frequencies, an intermediate frequency coupling network and an intermediate frequency amplifier, and a radio frequency feedback path connected between the output circuit of said frequency converter stage and said loop antenna circuit for producing regeneration at said radio frequencies, said intermediate frequency couplin network including a winding inductively coupled to said loop antenna circuit, said feedback path being constructed and arranged to provide maximum feedback at the low frequency end of the said band of frequencies, said feedback decreasing substantially as the high frequency end of the said band is approached.

5. A radio receiver of the superheterodyne type comprising in cascade, a resonant loop antenna circuit, said circuit having at least a loop ancoupled to a winding of few turns serially connected in said loop antenna circuit, aid feedback path being constructed and arranged to provide maximum feedback at the low frequency end of the said band of frequencies, said feedback decreasing substantially as the high frequency end of the said band is approached.

6. A radio receiver of the superheterodyne type comprising in cascade, a resonant loop antenna circuit, said circuit having at least a loop antenna winding and a variable condenser for tuning said circuit over a predetermined band of radio frequencies, a frequency converter stage providing an intermediate frequency signal which lies outside of and below said band of frequencies, an intermediate frequency coupling network and an intermediate frequency amplifier, a common source of automatic volume control voltage for said frequency converter stage and for said intermediate frequency amplifier, and a radio frequency feedback path connected between the output circuit of said frequency converter stage and said loop antenna circuit for producing regeneration at said radio frequencies, said control Voltage being applied to the signal input element of said converter stage through said radio frequency feedback path, said feedback path being constructed and arranged to provide maximum feedback at the low frequency end of the said band of frequencies, said feedback decreasing substantially as the high frequency of the said band is approached.

7. A radio receiver of the superheterodyne type comprising in cascade, a resonant loop antenna circuit, said circuit having at least a loop antenna winding and a variable condenser for tuning said circuit over a predetermined band of radio frequencies, a frequency converter stage providing an intermediate frequency signal which lies outside of and below said band of frequencies, an intermediate frequency coupling transformer and an intermediate frequency amplifier, a winding of few turns serially connected in said loop antenna circuit through a feedback path, said winding being placed in inductive relation to at least one of the windings of said transformer to feed back signals of radio frequency to the input of said converter stage in regenerative phase, said feedback path providing maximum feedback at the low frequency end of the said band of frequencies, and a source of automatic volume control voltage, said voltage being applied to a gain control element in said intermediate frequency amplifier, and also to the signal input element of said frequency converter through a path comprising, in series, said winding of few turns, said feedback path and said loop antenna.

8. A radio receiver according to claim 1 characterized in that some intermediate frequency energy is fed back from said intermediate frequency coupling network to the input circuit of said frequency converter stage by way of the said radio frequency feedback path, said intermediate frequency feedback being in a degenerative sense.

CHARLES TRAVIS. 

